Parasitic infections still represent one of the most important causes of morbidity and mortality worldwide. Among the parasites that cause human and animal pathology the phylum apicomplexa comprises a group of vector-borne parasites that is responsible for a wide variety of serious illnesses including but not limited to malaria, leishmaniasis and trypanosomiasis. Malaria alone infects 5-10% of humanity and causes around two milion deaths per year. [Schofield et al, “Immunological processes in malaria pathogenesis”, Nat Rev Imm 2005], [Schofiled L, “Intravascular infiltrates and organ-specific inflammation in malaria pathogenesis], [Mishra et al, “TLRs in CNS Parasitic infections”, Curr Top Micro Imm 2009], [Bottieau et al, “Therapy of vector-borne protozoan infections in nonendemic settings”, Expert Rev. Anti infect. Ther., 2011].
Toll-like receptors (TLRs) are germ-line encoded, phylogenetically ancient molecules that recognize evolutionary conserved structural relevant molecules (known as pathogen—associated molecular patterns (PAMPs)) within microbial pathogens. Various different cell types including cells of the immune system express TLRs and are thereby able to detect the presence of PAMPs. Sofar 10 functional TLR family members (TLR1-10) have been described in humans, all of which recognize specific PAMP molecules. Following recognition of these specific PAMPs TLRs induce and orchestrate the immuneresponse of the host to infections with bacteria, viruses, fungi and parasites. [Hedayat et al, “Targeting of TLRs: a decade of progress in combating infectious disease”, review, Lancet Infectious disease 2011], [Kwai et al, “TLRs and their crosstalk with other innate receptors in infection and immunity”, review, Immunity May-2011].
The immune system of the infected host responds to infection with the TLR induced production of pro-inflammatory cytokines mainly of the T-helper 1 type (Th1). While adequate amounts of these cytokines are beneficial and required to clear the infection an overproduction of these mediators is harmful to the host and associated with immune mediated pathology including neuropathology and tissue damage with severe and often fatal consequences. One prominent and highly relevant example of such immune mediated pathology is acute and cerebral malaria (CM) which causes severe clinical symptoms and is often fatal. [Schofield et al, “Immunological processes in malaria pathogenesis” Nat Rev Imm 2005], [Schofiled L, “Intravascular infiltrates and organ-specific inflammation in malaria pathogenesis], [Mishra et al, “TLRs in CNS Parasitic infections”, Curr Top Micro Imm 2009], [Bottieau et al, “Therapy of vector-borne protozoan infections in nonendemic settings”, Expert Rev. Anti infect. Ther., 2011] [Hedayat et al, “Targeting of TLRs: a decade of progress in combating infectious disease”, review, Lancet Infectious disease 2011]. Despite progress made in treatment and eradication of malaria, the mortality rate that is associated with severe malaria, including CM remains unacceptably high. Strategies directed solely at the eradication of the parasite in the host might therefore not be sufficient to prevent neurological complications and death in all cases of CM. Development of new innovative adjunct therapeutic strategies to efficiently reduce the CM-associated mortality and morbidity that is caused, in part, by host-mediated immunopathology remains therefore an urgent medical need [Higgins et al, “Immunopathogenesis of falciparum malaria: implications for adjunctive therapy in the management of severe and cerebral malaria”, Expert Rev. Anti Infect. Ther. 2011].
Recently further evidence has been provided that TLR9 plays a key role in the recognition and response to parasites including but not limited to Plasmodium, Leishmania, Trypanosoma and Toxoplasma [Gowda et al, “The Nucleosome is the TLR9-specific Immunostimulatory component of plasmodium falciparum that activates DCs”, PLoS ONE, June 2011], [Peixoto-Rangel et al, “Candidate gene analysis of ocular toxoplasmosis in Brazil: evidence for a role for TLR9”, Mem Inst Oswaldo Cruz 2009], [Pellegrini et al, “The role of TLRs and adoptive immunity in the development of protective or pathological immune response triggered by the Trypanosoma cruzi protozoan”, Future Microbiol 2011] and that interference with the activation of TLRs including TLR9 represents a promising strategy to prevent the deleterious inflammatory responses in severe and cerebral malaria [Franklin et al, “Therapeutical targeting of nucleic acid-sensing TLRs prevents experimental cerebral malaria”, PNAS 2011]
Malaria is an infectious disease caused by four protozoan parasites: Plasmodium falciparum; Plasmodium vivax; Plasmodium ovale; and Plasmodium malaria. These four parasites are typically transmitted by the bite of an infected female Anopheles mosquito. Malaria is a problem in many parts of the world and over the last few decades the malaria burden has steadily increased. An estimated 1-3 million people die every year from malaria—mostly children under the age of 5. This increase in malaria mortality is due in part to the fact that Plasmodium falciparum, the deadliest malaria parasite, has acquired resistance against nearly all available antimalarial drugs, with the exception of the artemisinin derivatives.
Leishmaniasis is caused by one or more than 20 varieties of parasitic protozoa that belong to the genus Leishmania, and is transmitted by the bite of female sand flies. Leishmaniasis is endemic in about 88 countries, including many tropical and sub-tropical areas. There are four main forms of Leishmaniasis. Visceral leishmaniasis, also called kala-azar, is the most serious form and is caused by the parasite Leishmania donovani. Patients who develop visceral leishmaniasis can die within months unless they receive treatment. The two main therapies for visceral leishmaniasis are the antimony derivatives sodium stibogluconate (Pentostam®) and meglumine antimoniate (Glucantim®). Sodium stibogluconate has been used for about 70 years and resistance to this drug is a growing problem. In addition, the treatment is relatively long and painful, and can cause undesirable side effects.
Human African Trypanosomiasis, also known as sleeping sickness, is a vector-borne parasitic disease. The parasites concerned are protozoa belonging to the Trypanosoma Genus. They are transmitted to humans by tsetse fly (Glossina Genus) bites which have acquired their infection from human beings or from animals harboring the human pathogenic parasites.
Chagas disease (also called American Trypanosomiasis) is another human parasitic disease that is endemic amongst poor populations on the American continent. The disease is caused by the protozoan parasite Trypanosoma cruzi, which is transmitted to humans by blood-sucking insects. The human disease occurs in two stages: the acute stage, which occurs shortly after infection and the chronic stage, which can develop over many years. Chronic infections result in various neurological disorders, including dementia, damage to the heart muscle and sometimes dilation of the digestive tract, as well as weight loss. Untreated, the chronic disease is often fatal. The drugs currently available for treating Chagas disease are Nifurtimox and benznidazole. However, problems with these current therapies include their diverse side effects, the length of treatment, and the requirement for medical supervision during treatment. Furthermore, treatment is really only effective when given during the acute stage of the disease. Resistance to the two frontline drugs has already occurred. The antifungal agent Amphotericin b has been proposed as a second-line drug, but this drug is costly and relatively toxic.
Toxoplasmosis is endemic through most of the world, which can infect a large proportion of the adult population. However, its prevalence differs in different countries. It is estimated to infect at least 10% of adults in northern temperate countries and more than half of adults in Mediterranean and tropical countries. Toxoplasma gondii is a ubiquitous, obligate intracellular protozoan and is considered to be the most common cause of infective retinitis in humans, which depends on a variety of factors, including climate, hygiene, and dietary habits. The course of disease in immunocompetent adults is usually asymptomatic and self-limiting. As soon as infection has occurred, the parasite forms latent cysts in the retina and in other organs of the body, which can reactivate years after the initial infection giving rise to acute retinochoroiditis and the formation of new retinochoroidal lesions. [Arevalo et al, “Ocular Toxoplasmosis in the developing world”, Internat. Ophthal. Clin 2010]
Neurocysticercosis is the most common parasitic disease of the CNS (incidence ˜2.5 million worldwide) caused by the larvae of Taenia solium. The disease has a long asymptomatic phase in humans characterized by the absence of a detectable inflammatory response surrounding the parasite. The overall immune response during the asymptomatic phase is of the Th2 phenotype. However, the destruction of larvae by therapeutic treatment or by normal parasite attrition causes a strong inflammatory response, often consisting of a chronic granulomatous reaction and manifestation of typical symptoms of the disease. The immune response in the CNS of symptomatic patients consists of an overt Th1 phenotype or a mixed Th1, Th2, and Th3 response, depending upon the absence or presence of granulomas. The hyperinflammatory response prevailing during the symptomatic phase in the CNS is responsible for the severe neuropathology and mortality associated with neurocysticercosis [Mishra et al, “TLRs in CNS Parasitic infections”, Curr Top Micro Imm 2009].
In view of the foregoing there is a strong need for the development of novel effective treatment options for the management of parasitic diseases including the diseases mentioned above, addressing especially the associated immunopathology.